A primary application for cryocoolers is with superconducting electronic devices. Superconductive electronic devices offer many advantages in terms of reduced power requirements, weight and size over conventional electronic devices. The phenomenon of superconductivity, however, only occurs at very low temperatures, usually within 20.degree. C. of absolute zero. It is for this reason that superconductor devices are conventionally cooled with liquid helium.
Superconductivity is characterized by a drastic reduction of electrical resistance in materials below a certain transition temperature. Above this temperature, superconductor material behaves conventionally and is said to be in the normal state. In their superconducting state, superconducting materials pass electrical currents very quickly and without the generation of heat because their electrical resistance is nearly zero. Superconducting devices, therefore, are able to transmit large amounts of energy without the losses which occur in conventional conductors such as copper. A corollary to this ability is the capability of some superconducting devices to detect minute amounts of electromagnetic energy that would be imperceptible with conventional devices.
A further property of many superconductor devices is their extreme sensitivity to magnetic radiation. Large magnetic fields not only can disrupt the output of superconductor devices but can cause them to revert to their conventional state. Therefore, superconductor devices must be operated at very low temperatures and in low magnetic fields.
A considerable number of superconductor devices with uniquely useful properties and a large range of potential applications have appeared over the last few years. Superconducting bolometers are capable of temperature measurements of a far more sensitive character than conventional devices. Similarly, superconducting Schottky diodes react much faster and produce far less heat than conventional room temperature diodes. Further, Josephson junction devices have, among numerous other applications, a prospective use as miniaturized computer logic elements.
Superconducting quantum interference devices (SQUID's) have been found to be among the most useful superconductor devices. They have been used as extremely sensitive magnetometers, galvanometers, susceptibility meters, radio frequency power meters and communication receivers. These SQUID devices are used in significant numbers by scientists in geology, medical research, and in the military. In all such uses, the users accept the inconvenience and cost of a liquid helium cryostat cooling system because the SQUID devices are simpler and more sensitive by several orders of magnitude than comparable conventional instruments.
Another electronic application requiring cryogenic cooling is operation of infrared (IR) radiation detectors. Heat energy masks the IR signal of faint or distant IR sources. The sensitivity of IR detectors is therefore greatly improved at cryogenic temperatures. Recently an expensive and highly successful American research satellite with an IR detector was abandoned due to the complete consumption of its liquid helium supply by its cryostat. Use of a low power cryocooler instead of a cryostat might have extended the life of this expensive satellite.
Liquid helium cryostats are inconvenient and expensive to use. This is because liquid helium is not readily available at many locations and, in all cases, must be carefully handled and secured. Further, liquid helium cryostats must be periodically serviced and replenished. The cost, inconvenience and attendant restrictions on cryostat cooled instruments are primary disadvantages in competition with conventional devices. If a way could be found to eliminate liquid helium cryostats, low temperature and superconducting electronic devices could be more widely utilized.
A cryocooler designed to replace conventional cryostats used with superconducting devices is described in U.S. Pat. No. 4,143,520 to Zimmerman. Zimmerman discloses a Stirling cycle cryogenic cooler with a nylon annular displacer housed in a glass reinforced plastic cylinder. Zimmerman utilizes these materials in order to give his device a low magnetic signature compatible with SQUID devices.
In the Zimmerman device, helium gas is driven between the displacer and the cylinder to form a gap regenerator between the helium source and the cold cylinder tip at cryogenic temperatures. While this represents an improved apparatus for cooling electronic devices, it has some practical disadvantages. A primary disadvantage is that the plastic and nylon material of the refrigerator are permeable to helium. Zimmerman's system therefore leaks helium from the gap regenerator over a period of time. This requires periodic recharging of the cryocooler. Also, the volume surrounding the refrigerator cylinder is held at a vacuum to minimize thermal leakage. With leakage of helium into that volume, the thermal leak increases unless a pump is used to maintain the vacuum. The plastic used by Zimmerman also has a tendency to appreciably deflect with the movement of the displacer. Deflection and movement of sensitive SQUID devices greatly degrades their accuracy.
A need, therefore, exists for an improved cryocooler for cryogenically cooled devices having a low magnetic signature and reduced maintenance requirements.